Top-Down Auditory Plasticity: Acceptable Noise Level Predicts and Reflects the Effect of Perceptual Learning in Experience-Induced Plasticity

Objective In the auditory system, tinnitus and superior speech perception in noise are examples of negative and positive plasticity that can result from sensory neural hearing loss and life experiences dealing with more complex stimuli and learning, respectively. The main objective of this study was to determine the relationship between acceptable noise level (ANL) values and perceptual learning in individuals exposed to unavoidable occupational noise. Materials & Methods Here we document a form of plasticity in top-down auditory pathways through the measurement of the acceptable noise level in 60 adults, 27 females and 33 males, with normal hearing (Amiraalam state Hospital, Tehran, Iran 2016). Individuals were assigned to one of two groups: those with and without the occupational experience of speech perception in noise. Results The test group had statistically significant lower acceptable noise level and significantly higher background noise level scores compared with the control group. Conclusion Using acceptable noise level, we attributed differences in individuals’ abilities to tolerate varying amounts of background noise and speech perception in noise function to the auditory efferent system. Working in crowded locations due to job nature can influence differences in speech perception in noise function.


Introduction
Speech perception in noise is one of the most complex mental activities encountered in everyday life and is dependent upon the optimal functioning of peripheral hearing, central auditory processing and cognition, and individual daily experiences (1).
The auditory cortex is organized through the experiences of environmental sounds (2), and these experiences influence the development of differences among people in the ability to process auditory sounds (3). Perceptual learning in adults can be strongly attributed to one's level of attention, physical and mental activity, and social situations (1,3,4). The effects of these factors have been researched in the plasticity of central auditory processing, which results in improved speech perception capabilities under noisy conditions. In other words, the ability of the auditory efferent system to suppress undesirable auditory inputs is an important effect that has an active role in signal processing and modulating auditory input stimulants (4). fibers make contact with the outer hair cells, thus modulating active amplification mechanisms (4,5).
The function of the MOCB is to reduce the number of reflexive responses to sounds (5), suppress basilar membrane response (6), and consequently, protect the ear against extremely loud sounds (5). In addition, the MOCB facilitates speech perception in noise and improves signal-to-noise ratio and selective attention (5,(7)(8)(9). This ability contributes to the enhancement of the signal-tonoise ratio through the reduction of the auditory nerve response to noise input and an increase in the auditory nerve response to transient sounds (6,7,10). A superior ability of some individuals was shown to extract signals from noise. This phenomenon is derived from forceful perceptual learning (4). We assumed that individuals with long-term experience of speech perception in noise conform to this fact.
One of the tests in the field of speech-in-noise is the acceptable noise level (ANL) test, and its ability to assess the auditory efferent system has been examined. The ANL test can reveal the function of the ability of the auditory efferent system to process signals in noise (11,12). The ANL test was introduced as a procedure for defining acceptable noise levels while people listen to a speech (13). The The main objective of this study was to determine the relationship between ANL values and perceptual learning in individuals exposed to unavoidable occupational noise. We asked if there would be any differences between ANL scores in people who work in noisy environments and those who do not.

ANL Measurement
A loudspeaker was placed 1 m in front of each individual at 0° Azimuth). The Persian version of the ANL test was used in this study; this version has been used in a previous study (19)(20)(21). A running speech with a female voice telling a story was used. The noise of 12 people babbling was used as the test noise (19)(20)(21). The test was adequately explained to each participant before beginning.
ANL score is obtained in three steps: Firstly, a running speech is heard from a loudspeaker as it rises and falls; this yields the person's MCL.
Babble noise is then added to the speech and is increased until the person can no longer tolerate it without any annoyance or discomfort. Lastly, the ANL score is obtained by subtracting MCL from BNL: ANL = MCL -BNL.

Statistical Method
The Kolmogorov-Smirnov test was used for assessing the normal distribution of data. Pearson's correlation was used to assess the relationships among variables, and the independent t-test was used to compare data between groups. All analyses were performed using SPSS version 16 (ver.16.0. Chicago, IL, USA).

Results
There were no statistically significant differences between the ages of the groups (  There was a slight inverse relationship between BNL and ANL scores in Group 2 only (r=−0.579, P=0.000). There was a significantly strong relationship between years of experience and ANL scores (r = 0.95, P=0.000).

Discussion
Our data demonstrate that ANL in people who work in noisy environments is superior to those There was a slight inverse relationship between  In individuals with perfect hearing, efferent fibers suppress the basilar membrane responses in the region of the cochlea that is most sensitive to low frequencies (6). The amount of auditory efferent activity is proportional to ambient noise level (6).